How Do You Find a Planet With No Home?

Update: NASA’s Kepler spacecraft has entered emergency mode. The mission team has declared a spacecraft emergency giving them priority access to NASA’s Deep Space Network. NASA isn’t sure what caused the emergency, but I’ll update the post as we hear more.

A planet moves between us and its host star, barely dimming its star. But it’s enough for scientists to figure out several characteristics about the planet including its size and orbit. This is called the transit method and has been the go-to way for spotting exoplanets for years.

These exoplanets tend to be located right beside their stars. Orbits (years) take just hours in some instances. Days and weeks in others. But what about exoplanets that don’t have a home? Or, planets that sit further away from their star? How can scientists find the planets that just wander through the cosmos?

NASA’s Kepler space telescope is entering a new stage of its mission this week. Kepler, along with telescopes on the ground, will begin surveying millions of stars located towards the heart of the Milky Way Galaxy. And while they are at it, search for exoplanets that aren’t near stars.

Kepler will hunt for stars with a temporary increase in brightness

Wait, I thought scientists spot exoplanets by looking for a decrease in a star’s brightness? Yep, but they can use a different method that involves an increase in brightness. Gravitational microlensing. Most of us have seen gravitational lensing used to teases out details of faraway galaxies. Kepler is about to use it to find planets.

Kepler will be looking for gravity’s influence on the light of a distant star. If a large planet moves in the space between Kepler and a distant star, its gravity will warp the light from the background star. Gravity from the planet acts as a lens. The effect is much smaller than what we see from distant galaxies, but the principal is the same.

Here’s how it will work with planets.

The window to see lensing from an exoplanet is small. But Kepler is perfectly suited for it. Because Kepler continuously monitors these stars, it will be ready for any changes that happen. Scientists expect these microlensing events to last just a day or two at most.

“We are seizing the opportunity to use Kepler’s uniquely sensitive camera to sniff for planets in a different way,” said Geert Barentsen, a research scientist at NASA’s Ames Research Center.

Multiple telescopes on the ground will measure these microlensing events and help pinpoint exactly where the planet in the foreground using parallax. NASA explains parallax in a simple experiment you can do right now.

To understand parallax, extend your arm and hold up your thumb. Close one eye and focus on your thumb and then do the same with the other eye. Your thumb appears to move depending on the vantage point. For humans to determine distance and gain depth perception, the vantage points, our eyes, use parallax.

Kepler will keep a constant eye towards the center of the Milky Way galaxy for 80 days. Scientists are hoping to witness at least 100 lensing events. As many as 10 or more could be exoplanets “occupying relatively unexplored regimes of parameter space.”

Finding an exoplanet via gravitational microlensing has been done before. But the next few months will be the first time observations from Kepler and the ground will be able to pinpoint where these wandering planets are.

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